1. Field of the Invention
This invention relates to steam turbine power plants, and more particularly, to a nuclear steam power plant incorporating a pressure-increasing device in the steam system.
2. Description of the Prior Art
It is well known to those skilled in the art of steam power generation that nuclear steam turbine systems with saturated or superheated steam conditions usually employ reheating to improve the thermal efficiency of the system. In most cases, throttle steam alone, or in conjunction with partially expanded steam from a high pressure turbine extraction zone, is used to superheat the high pressure turbine exhaust flow after it has been dried in a moisture separator.
It is also well known to those skilled in the art of steam power generation that a plurality of feedwater heaters are usually employed to increase the temperature of a condensate taken from a condenser element within the steam power system before that condensate is reintroduced into a primary steam generator section. Increasing the temperature of the condensate prior to reintroduction into the primary heater section increases the overall efficiency of the power plant. It has been the practice in the art to heat the condensate in the feedwater heater through the use of high pressure turbine element. Of course, lower pressure feedwater heaters may be employed in series between the condenser and the final feedwater heater to provide gradual step increases of the condenser temperature prior to its introduction into the primary steam generator section.
It is the general practice in the art to cascade the condensed throttle steam found in the reheater drains into the highest pressure feedwater heater. The usual pressure difference between the pressure within the reheater drains and the pressure of the extraction steam introduced into the final feedwater heater is approximately 500 psi. The reheater drains are introduced into the final feedwater heater as an alternative to introduction of the reheater drains into the discharge conduit of the final feedwater heater. However, the possibility that the condensed throttle steam from the reheater drain would flash, that is, pass from the liquid to the gaseous state due to the temperature increase at the decreased ambient pressure, resulted in abandonment of this approach. In order to avoid flashing, and the concomitant cavitation engendered within the discharge conduit of the final feedwater heater, the condensed throttle steam from the reheater drains are introduced into the shell of the final feedwater heater. It is obvious, however, that introduction of the condensed throttle steam from the reheater drain having a pressure energy approximately 500 psi higher than the pressure of the extraction steam represents a dissipation of available energy within the system.
The feedwater heater capacity to transfer heat to the condensate passing therethrough is directly dependent upon the temperature of the extraction steam used as a heating medium. In turn, the temperature of the extraction steam depends upon the pressure level within the extraction steam. It therefore follows that an increase in the pressure of the extraction steam will increase the temperature of the extraction steam used as a heating medium in the feedwater heater. Thus, higher pressure extraction steam will directly result in increased heating capacity within the feedwater heater.
Extraction of steam from the high pressure turbine elements at a point in the high pressure turbine closer to the inlet orifice than is presently being used is one directly available source of higher pressure extraction steam. It is obvious that tapping steam within the high pressure turbine element at an earlier point will provide a higher extraction steam pressure. However, a disadvantage to such a relocation of the extraction zone is that the overall output or work of the power plant would be lowered since steam capable of producing useful work is diverted from the system.